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The leader of Islamic State's branch in Afghanistan and Pakistan was killed in a U.S. drone strike on July 26, a Pentagon spokesman said on Friday after the Afghan ambassador to Pakistan announced the news to Reuters.

The death of Hafiz Saeed Khan is a blow to efforts by Islamic State - also known as ISIS or Daesh - to expand from its heartlands in Syria and Iraq into Afghanistan and Pakistan, already crowded with jihadist movements including the Taliban and al Qaeda.

It is the second U.S. killing of a prominent militant in the region in months. In May, a U.S. drone killed Afghan Taliban leader Mullah Akhtar Mansour in a strike in Pakistan.

Despite that, Afghanistan's 15-year-old war grinds on with no clear victory in sight. Taliban fighters have been threatening at least two provincial capitals this summer, in Helmand and Kunduz, and a U.S. government report said Afghan forces have lost 5 percent of territory this year.

In terms of its own territory, Islamic State has been largely confined to a handful of districts in Afghanistan's Nangarhar province, which borders Pakistan, where IS militants - mostly defectors from the Taliban - are blamed for raiding villages and government outposts.

Still, worries that Islamic State might be expanding its operational reach heightened this week when the group took credit for an attack on a Pakistani hospital that killed at least 74 people in the southwestern city of Quetta. A Pakistani Taliban faction also claimed responsibility.

A few weeks earlier, Islamic State claimed an attack on a rally in Kabul that killed more than 80 people.

Bitter rivals

Khan has been reported dead before. But a claim by Afghan intelligence agents last year that he had been killed was never confirmed.

On Friday, however, Afghan Ambassador Omar Zakhilwal told Reuters he had seen confirmation from Afghan security forces.

"I can confirm that ISIS Khurasan (Afghanistan and Pakistan) leader Hafiz Saeed Khan along with his senior commanders and fighters died in a U.S. drone strike on July 26 in Kot district of Afghanistan's Nangharhar province," he said.

Pentagon spokesman Gordon Trowbridge confirmed Khan's death, and said in a statement that the air strike took place during joint operations by U.S. and Afghan special operations forces against IS in the southern part of Nangarhar province. Trowbridge said the airstrike was in Achin district, as opposed to Kot district.

The Taliban's various factions in Afghanistan and Pakistan as well as their al Qaeda allies are bitter rivals of Islamic State's al-Baghdadi. The Taliban reject al-Baghdadi as leader of an envisioned worldwide caliphate.

In Afghanistan, Taliban and Islamic State fighters have battled over territory in Nangarhar, though both have recently been more busy defending against U.S. and Afghan assaults.

Between January and early August, American warplanes conducted nearly 140 air strikes against Islamic State targets in Afghanistan, according to the U.S. military.

Afghan forces, backed by the American military, killed an estimated 300 Islamic State fighters in an operation mounted two weeks ago, the top U.S. and NATO commander in Afghanistan said on Wednesday, calling it a severe blow to the group.